Ink jet coatings that are waterfast or waterproof are highly desirable in the ink jet printing industry. In general, there are two methods in the prior art for making an ink jet coating waterfast or waterproof. The first involves using a water-insoluble binder, such as a latex or a cross-linked polymer (e.g. cross-linked polyvinyl alcohol). By using a water-insoluble binder, the coating will maintain its strength when exposed to water. This concept applies both to coatings that are composed of only a binder (polymer) component and to coatings that are composed of binder plus pigment. This is the approach that is described in U.S. Pat. No. 4,877,680 to Sakaki et al. as well as U.S. Pat. No. 5,270,103 to Oliver et al.
However, since most dye fixatives and most dyes or inks are water-soluble, even if the binder is water-insoluble, the dye, which forms a printed image on the paper, will still have a tendency to run or bleed if it makes contact with water. Thus, water-insoluble binders should only be used if the dye fixative/dye complex is also water-insoluble.
The second method involves using a dye fixative of high molecular weight. When the paper is imaged, the dye from the ink jet ink can complex with the dye fixative, thereby decreasing the solubility of the dye and increasing its water resistance. This method can be used if the fixative is water-soluble or insoluble. However, for best water resistance, if the fixative is water-soluble, it should also have a sufficient number of absorptive sites so that the resulting dye fixative/dye complex is water-insoluble. This approach is described in Sugiyama et al in U.S. Pat. No. 4,371,582.
Because this method requires using a dye fixative of high molecular weight, and in some case, with a sufficient number of absorptive sites, the selection of available dye fixatives is significantly reduced. The high molecular weight will also make the ink jet coating more viscous, and hence more difficult to pump and coat during the manufacturing process.
A need therefore exists to provide a more cost effective and efficient way to make ink jet recording sheets with excellent waterfast properties.
The present invention is an improvement over the prior art methods for making waterfast ink jet recording sheets. In the present invention, it is now possible to use dye fixatives which were once regarded as too water-soluble to make suitable waterfast recording sheets. The present invention comprises compositions containing blends of dye fixatives. The blends form water-insoluble complexes. The dye or ink will adhere to the water-insoluble complex and will not wash off the surface of the substrate.
The compositions are absorbed into the surface of the substrate as a sizing material, or alternatively form a continuous layer on the substrate. The resulting waterfast ink jet recording sheets made using these compositions are therefore an important improvement over the prior art.
Moreover, dye fixatives of the prior art are commonly used individually. However, because ink jet printers use different inks, an individual dye fixative is generally not suitable for all types of inks. As stated above, the present invention uses compositions containing a blend of at least two dye fixatives. By using a blend of dye fixatives, a wider range of ink types can be made waterfast, and hence are available when printing images using ink jet printers.
Additionally, the compositions of the present invention are more easily mixed with starch as a binder or sizing agent, because the pH of the blend of dye fixatives is adjusted to be within the most favorable pH range for starch to remain stable, namely 6 to 8. Since starches are a common and inexpensive sizing and binding material for paper and paper coatings, improved compatibility with starches greatly enhances ease of use on the paper and coating machines used in the industry.
Accordingly it is the broad object of the present invention to provide a coating composition and an ink jet recording sheet having improved waterfast qualities.
Additionally, it is another object of the present invention to provide a coating composition for use in an ink jet recording sheet having unexpected enhanced print performance, i.e., an increase in print density and print resolution along with reduced wicking and color-to color bleed.
It is another object of the present invention to provide a coating composition and an ink jet recording sheet, which can be used with a wider variety of inks.
It is a further object of this invention to provide an ink jet recording sheet that is more cost effective and easier to manufacture than the prior art ink recording sheets.
It is yet another object of the present invention to provide a method for making an ink jet recording sheet to create these improved ink jet recording sheets and images.